(a) Field of the Invention
The present invention relates to a circuit which, by the use of a plurality of associated microphones, compensates for abnormal output characteristics of one of these microphones by the output characteristics of the other microphones involved. More particularly, the present invention utilizes a plurality of microphones having different acoustic-to-electric characteristics, and aims to obtain excellent overall acoustic-to-electric characteristics by making positive use of the advantages of the respective microphones employed.
(b) Description of the Prior Art
As is well known, microphones vary in type, including the type represented by the acoustic-to-mechanical transducing system and the type represented by the mechanical-to-electric transducing system. These known microphones are appropriately selected in actual use in accordance with the external conditions such as the condition of the source of sound being picked up and the condition of the place where the sound is picked up.
From the viewpoint of directional characteristics, there have been often used, in general, such directional microphones as the so-called bidirectional microphones, and the so-called unidirectional microphones. Such directional microphones have been used to achieve clear reception of the sound generated from the source thereof, and to prevent the occurrence of howling (acoustic feedback) phenomenon which could arise when the sound picked up is being reproduced simultaneously as the sound is being picked up, to thereby monitor the reproduced sound through a monitor speaker.
The aforesaid directional microphones have been realized usually by the microphones of either one of the following two types. One of them is a pressure gradient type microphone in which both sides of a diaphragm are exposed to the sound-generating source so that the diaphragm is driven by the difference in the pressures of sound which act on both sides of the diaphragm. The other of them is the so-called phase-shifting type microphone arranged so that the front side of the diaphragm is exposed directly to the sound source, and that an acoustic phase-shifting circuit is provided on the rear side of the diaphragm, to thereby perform the dual operations exerted by the so-called pressure type microphone and by the pressure gradient type microphone.
These known directional microphones are such that, because of the so-called proximity effect, when such microphone is moved closer to the source of sound, its sensitivity in the low frequency range of signal is elevated. Therefore, these known directional microphones have the drawback and inconvenience that, owing to the reason as mentioned above, there arises a variation in the quality of sound being recorded or picked up in accordance with the distance between the microphone and the source of sound. Thus, efforts have been made in the past to suppress the specific rise of sensitivity of microphone in the low frequency range of sound attributable to said proximity effect by providing acoustic circuit devices in the transducer, but with no ultimate successful achievement in the elimination of proximity effect. In order to compensate for such inconveniences, many of the prior techniques have employed the consideration that, in order that a sound may be picked up or recorded in better quality, a directional microphone such as a bidirectional microphone or a unidirectional microphone which is to be used at a position close to the source of sound is designed so that the sensitivity to the low frequency range of sound is lowered to a certain degree in view of the anticipated rise which would take place in its sensitivity to such low frequency range of sound when the microphone is used at a position close to the source of sound. On the other hand, in case a microphone is not provided with such consideration, there is additionally attached to the microphone system a sound quality adjustment device such as a tone-effector device or a tone control device for allowing the user to externally adjust the sensitivity of the microphone in the low frequency range, to thereby compensate for the imbalance of the frequency of the sound being recorded or picked up, by preliminarily lowering the response to the low frequency range of sound when the microphone is used in substantial proximity to the source of sound. Even through these various efforts of the prior techniques, it has been impossible to completely avoid such changes in quality of sound as are attributable to the distances of the microphone to the source of sound in use.
Such proximity effect, however, will not take place in the so-called nondirectional type microphones which are actuated by the pressure of a sound applied. Nevertheless, this nondirectional microphone is inferior to the above-mentioned directional microphones such as the pressure gradient type microphone or the phase-shifting type microphone, in performing clear recording or capture of a sound as well as in the prevention of howling at the time of simultaneous reproduction of the sound being picked up.
From the general point of view, both the pressure gradient type microphones and the phase-shifting type microphones can be termed as still being superior to the nondirectional microphones with respect to the quality and articulation of the sound being picked up or recorded and also to the prevention of howling, although the former two types have drawbacks resulting from the proximity effect.